LACTATE AND PYRUVATE ISOTOPIC ENRICHMENTS IN PLASMA AND TISSUES OF POSTABSORPTIVE AND STARVED RATS

It has been proposed that plasma pyruvate isotopic enrichment (IE) dur ing infusion of labeled lactate could be used to estimate the intracel lular IE of lactate and pyruvate and thus to calculate their turnover rate. We determined the relations of plasma and tissue IE of lactate a nd pyruvate in anesthetized rats infused with [3-C-13]lactate in an ar tery and sampled from a vein (A-V mode) or infused in a vein and sampl ed from an artery (V-A mode). In both groups of rats, the ratio of tis sue to plasma lactate IE was < 1 with large differences between tissue s: the highest ratio was observed in heart and the lowest in soleus. W ith the exception of liver, this ratio was higher in the A-V than in t he V-A mode. Pyruvate IE was lower than lactate IE in tissues, with a few exceptions, and in plasma. This ratio of pyruvate to lactate IE wa s similar to 0.70 in plasma in A-V and V-A modes. Moreover pyruvate IE was also always higher in plasma than in tissues. This seemingly surp rising result could be explained by the production of labeled pyruvate from labeled lactate inside the circulation by erythrocytes, because we observed a rapid isotopic equilibrium between lactate and pyruvate in blood ''in vitro.'' Apparent lactate turnover was higher in the A-V than in the V-A mode when it was calculated using lactate as well as pyruvate IE. Therefore plasma pyruvate IE cannot be used in rats to es timate tissue IE and did not reconcile turnover rates measured using t he A-V or V-A mode. Comparison of these results in rats with data obta ined in monkeys and theoretical considerations suggest that the degree of isotopic equilibration between lactate and pyruvate and between pl asma and tissues depends mainly on lactate and pyruvate fractional rep lacement rates.